Remove remaining uses of null's absolute address from non-IA32.

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index 5382e88b801b87f7f2a78d02f393a8365b818f84..19fc8b3fcf4bb8883bde34513525de0706d05776 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -22,6 +22,7 @@
 
 namespace dart {
 
+DECLARE_FLAG(bool, allow_absolute_addresses);
 DECLARE_FLAG(bool, emit_edge_counters);
 DECLARE_FLAG(int, optimization_counter_threshold);
 DECLARE_FLAG(bool, throw_on_javascript_int_overflow);
@@ -2600,6 +2601,7 @@ class CheckStackOverflowSlowPath : public SlowPathCode {
       Register temp = instruction_->locs()->temp(0).reg();
       __ Comment("CheckStackOverflowSlowPathOsr");
       __ Bind(osr_entry_label());
+      ASSERT(FLAG_allow_absolute_addresses);
       __ LoadImmediate(temp, Immediate(flags_address));
       __ movq(Address(temp, 0), Immediate(Isolate::kOsrRequest));
     }
@@ -2646,7 +2648,7 @@ void CheckStackOverflowInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
 
   Register temp = locs()->temp(0).reg();
   // Generate stack overflow check.
-  if (compiler->is_optimizing()) {
+  if (compiler->is_optimizing() && FLAG_allow_absolute_addresses) {
     __ LoadImmediate(
         temp, Immediate(Isolate::Current()->stack_limit_address()));
     __ cmpq(RSP, Address(temp, 0));
@@ -3405,9 +3407,7 @@ void UnboxInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
 
     if ((value()->Type()->ToNullableCid() == box_cid) &&
         value()->Type()->is_nullable()) {
-      const Immediate& raw_null =
-          Immediate(reinterpret_cast<intptr_t>(Object::null()));
-      __ cmpq(box, raw_null);
+      __ CompareObject(box, Object::null_object());
       __ j(EQUAL, deopt);
     } else {
       __ testq(box, Immediate(kSmiTagMask));